1. Field of the Invention
The present invention relates to a multi reader head of a hard disk drive (HDD).
2. Description of the Related Art
In order to enhance area density capacitance (ADC) in recent HDDs, a multi reader head having a plurality of readers is being developed. The reader includes a magneto-resistive effect element (MR element), such as a tunnel magneto-resistance effect (TMR) element or a giant magneto-resistance effect (GMR) element, as a primary component.
A head with a configuration where two MR elements that overlap via a common bias layer are interposed between a pair of shields is disclosed in U.S. Pat. No. 6,819,530B2. A head with a configuration where two MR elements overlap via a common antiferromagnetic layer is disclosed in US2003/0011939A1. In these configurations, two readers share a bias layer or an antiferromagnetic layer and are not independent from each other. A configuration where two readers, which have a structure where an MR element is interposed between a pair of shields, and are independent from each other and overlap via an insulating layer, is disclosed in US2011/0216432A1. In such a configuration where a plurality of readers that are independent from each other overlap via an insulating layer, in the case when each reader is composed of a current-perpendicular-to-plane (CPP) type TMR element or GMR element where a current flows in a direction perpendicular to the principal plane of each layer, and there is a positional relationship between a reader that is situated at the lower side in a lamination direction and another reader that is situated at the upper side in the lamination direction, and accuracy of the width and height of each reader is very important, and sophisticated processing and control are required. In particular, it is important to reduce the distance between the readers (RRS: Reader-Reader Separation) affecting the reading accuracy of adjacent tracks as much as possible, and such reduction is a key factor for obtaining a characteristic of a multi reader head. Furthermore, the RRS is the distance between center lines of free layers in the lamination direction in the MR elements of readers, which are adjacent in the lamination direction.
As described above, in a multi reader head where a plurality of readers overlap, it is necessary to reduce the RRS to, for example, approximately several tens of nm. As shown in FIGS. 1A and 1B, if a lower shield and an upper shield that interpose an MR element V in a reader R1 situated on a lower side are indicated as S1 and S2, respectively, and a lower shield and an upper shield that interpose an MR element V in a reader R2 situated on an upper side are indicated as S3 and S4, respectively, in order to reduce the RRS as much as possible, it is desirable to reduce the distance D23 between the reader R1 and the reader R2, i.e., the distance D23 between the shield S2 and the shield S3, the distance D12 between the lower shield S1 and the upper shield S2 of the reader R1 and the distance D34 between the lower shield S3 and the upper shield S4 of the reader R2, respectively. Furthermore, in the entire specification, regardless of a posture or the like at the time of using the multi reader head, the lamination direction of the plurality of readers is defined as the upward/downward direction, and one side in the lamination direction is indicated as “upper” and the other side is indicated as “lower.” Specifically, the relationship between “upper” and “lower” is defined such that the reader formed first in the manufacturing process of the multi reader head is referred to as “lower-side reader” and the reader that is laminated and formed later is referred to as “upper-side reader.” Even when “upper” and “lower” are indicated relative to each member within each reader and various members other than the reader, these are expressed based upon the definition set forth here.
Further, since the number of rotations of the HDD becomes greater and the recording bit size becomes smaller in association with the improvement of recording density in recent HDDs, a high-frequency noise component is easily generated. In order to eliminate the high-frequency noise component, it is preferable to reduce capacitance between each shield of the readers as much as possible. Then, the capacitance between the shields decreases as the distance between the shields increases.
In other words, the reduction of the RRS as much as possible in order to enhance the reading accuracy of adjacent tracks and the increase in the distance between the shields as much as possible in order to reduce the capacitance between the shields are a trade-off. A countermeasure to reduce the capacitance in the case of consistent RRS by changing the material of the insulating layer situated between the shields from common aluminum oxide (for example, Al2O3) to silicon oxide (for example, SiO2) with a smaller permittivity may be considered. However, the capacitance between the shields cannot be sufficiently reduced only by changing the material of the insulating layer. In particular, since the capacitance between the readers, which did not exist in a head with only a single reader, is an issue in a multi reader head, it is desirable to reduce this capacitance as much as possible.